Three rare Persian leopard cubs were unveiled on Friday at Lisbon zoo, part of a European breeding programme for the endangered species. The three male cubs were born in the Portuguese capital in May but their arrival was not previously announced."Among 44 zoos participating in the European breeding programme, there have been births every year," said Jose Dias Ferreira who directs the programme."But for us (Lisbon zoo) it's our first arrivals in seven years," he added.Until the mid-20th century, the Persian leopard was widely distributed over the mountainous areas across the Caucasus. However its population has since dramatically declined, or disappeared altogether in many areas, mainly through human activities.A programme of reintroducing the leopards to the Russia Northern Caucasus began in 2016.Russia is a partner in the programme along with the International Union for Conservation of Nature (IUCN).The three leopard cubs at Lisbon zoo will stay in their enclosure with their mother until they are 18-24 months old. Then they will probably be sent on to other zoos in the breeding programme.There are only around a thousand of the big cats still living in the wild, according to Dias Ferreira. Some 400 of these are in Iran, with other populations in Armenia, Georgia and Azerbaijan.